The Thin Film Transistor Liquid Crystal Display (TFT-LCD) possesses advantages of thin body, power saving and no radiation to be widely used in many application scopes. Most of the LCDs on the present market are back light type liquid crystal display devices, which comprise a liquid crystal display panel and a back light module. Generally, the liquid crystal display panel comprises a Color Filter (CF) substrate, a Thin Film Transistor (TFT) substrate, a LC (Liquid Crystal) sandwiched between the CF substrate and the TFT substrate and sealant. The working principle of the liquid crystal display panel is to locate liquid crystal molecules between two parallel glass substrates. The light of back light module is reflected to generate images by applying driving voltages to control whether the liquid crystal molecules to be changed directions.
In the liquid crystal display panel, a layer of alignment film is also required to be formed on the thin film transistor substrate and a color film substrate in generally. After the alignment film contacts with the LC, it can make the LC have a pre-tilted angle in certain direction, and thus provide a loading angle (the pre-tilted angle has significant influence to the driving voltage, contrast, response time and view angle of the TFT-LCD) for the liquid crystal molecules. Polyimide (PI) is commonly chosen to be the material of the alignment film, which mainly has rubbing alignment type IP material and lighting alignment type IP material. However, either of the alignment materials has its own drawback. The rubbing alignment type IP material is to form the alignment film with Rubbing method. The Rubbing method is to use fabric roller to perform contacting directional mechanical friction on the macromolecular PI film surface. The energy provided by rubbing the macromolecular surface make the macromolecular main chain be aligned in direction due to the extension, and thus the interaction of the branch and the LC is controlled to align the LC in the direction of the pre-tilted angle; as rubbing, the issues of powder particles, residual electrostatic and brush marks reduce the process yield. The lighting alignment type IP material is to form the alignment film with photo-alignment technology. The photo-alignment technology is to utilize the photochemical reaction of the ultraviolet light polymer monomers to create the anisotropy, and the liquid crystal molecules and the alignment film surface branch interact with each other. For reaching the stable status of the minimum energy, the liquid crystal molecules are aligned along the direction of which the acted force is the maximum defined by the lighting alignment. The lighting alignment type IP material can solve the aforesaid problems but with the restriction of the material properties, the heat resistance and the aging resistance are poor, and meanwhile, the LC anchoring ability is weaker, the quality of the panel is influenced. Besides, the PI material itself has high polarity and high water absorption. The storage and the delivery can easily change the property to result in the nonuniform alignment. The price of the PI material is expensive, and the film formation process on the TFT-LCD is more complicated, which leads to the increase of the panel cost.
Besides, for the LCD in the mainstream market, three types, which respectively are Twisted Nematic (TN), In-Plane Switching (IPS) and Vertical Alignment (VA) can be illustrated. The VA liquid crystal display possesses extremely high contrast than the liquid crystal displays of other types. It has very wide application in large scale display, such as television or etc. The polymer-stabilized vertical alignment (PSVA) technology can make the liquid crystal display panel possess advantages of faster response time and high transmittance. The distinguishing feature is to form polymer protrusions on the surface of the alignment film with the reactive monomers in the liquid crystal material by ultraviolet light irradiation to make the liquid crystal molecules have a pre-tilted angle.